linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <69621b58-5142-48ea-9dd8-6baed69e50f8@linux.dev>
Date: Wed, 24 Sep 2025 19:47:11 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, dev.jain@....com, hughd@...gle.com,
 ioworker0@...il.com, kirill@...temov.name, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mpenttil@...hat.com, npache@...hat.com,
 ryan.roberts@....com, ziy@...dia.com, richard.weiyang@...il.com,
 akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap
 entries



On 2025/9/24 18:10, David Hildenbrand wrote:
> On 24.09.25 12:02, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> The existing check in hpage_collapse_scan_pmd() is specific to uffd-wp
>> markers. Other special markers (e.g., GUARD, POISONED) would not be 
>> caught
>> early, leading to failures deeper in the swap-in logic.
>>
>> hpage_collapse_scan_pmd()
>>   `- collapse_huge_page()
>>       `- __collapse_huge_page_swapin() -> fails!
>>
>> As David suggested[1], this patch skips any such non-swap entries early.
>> If a special marker is found, the scan is aborted immediately with the
>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>> work.
> 
> Note that I suggested to skip all non-present entries except swap 
> entries, which includes migration entries, hwpoisoned entries etc.

Oops, I completely misunderstood your suggestion :(

It should be to handle all special non-present entries (migration,
hwpoison, markers), not just a specific type of marker ...

How about this version, which handles all non-swap entries as you
suggested?

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 7ab2d1a42df3..27f432e7f07c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1284,7 +1284,23 @@ static int hpage_collapse_scan_pmd(struct 
mm_struct *mm,
         for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
              _pte++, addr += PAGE_SIZE) {
                 pte_t pteval = ptep_get(_pte);
-               if (is_swap_pte(pteval)) {
+               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+                       ++none_or_zero;
+                       if (!userfaultfd_armed(vma) &&
+                           (!cc->is_khugepaged ||
+                            none_or_zero <= khugepaged_max_ptes_none)) {
+                               continue;
+                       } else {
+                               result = SCAN_EXCEED_NONE_PTE;
+                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+                               goto out_unmap;
+                       }
+               } else if (!pte_present(pteval)) {
+                       if (non_swap_entry(pte_to_swp_entry(pteval))) {
+                               result = SCAN_PTE_NON_PRESENT;
+                               goto out_unmap;
+                       }
+
                         ++unmapped;
                         if (!cc->is_khugepaged ||
                             unmapped <= khugepaged_max_ptes_swap) {
@@ -1293,7 +1309,7 @@ static int hpage_collapse_scan_pmd(struct 
mm_struct *mm,
                                  * enabled swap entries.  Please see
                                  * comment below for pte_uffd_wp().
                                  */
-                               if (pte_swp_uffd_wp_any(pteval)) {
+                               if (pte_swp_uffd_wp(pteval)) {
                                         result = SCAN_PTE_UFFD_WP;
                                         goto out_unmap;
                                 }
@@ -1304,18 +1320,6 @@ static int hpage_collapse_scan_pmd(struct 
mm_struct *mm,
                                 goto out_unmap;
                         }
                 }
-               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-                       ++none_or_zero;
-                       if (!userfaultfd_armed(vma) &&
-                           (!cc->is_khugepaged ||
-                            none_or_zero <= khugepaged_max_ptes_none)) {
-                               continue;
-                       } else {
-                               result = SCAN_EXCEED_NONE_PTE;
-                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
-                               goto out_unmap;
-                       }
-               }
                 if (pte_uffd_wp(pteval)) {
                         /*
                          * Don't collapse the page if any of the small
---

Thanks,
Lance